This application claims priority from U.S. Provisional Application 60/285,377, filed Apr. 20, 2001.
The present invention relates to a functional fluid, such as a tractor hydraulic fluid, which exhibits shear stable viscosity characteristics and has a wide temperature range of operation.
Requirements for physical characteristics of functional fluids, including lubricating oils, are becoming more stringent. Equipment manufacturers want lower viscosities at low temperatures (i.e. −40° C.) while maintaining high temperature (100° C.) thickening. They also want the oil to be more shear stable, that is, to maintain its viscosity after shearing. The present invention relates to the use of mixtures of viscosity modifier types that produce an improvement in these physical properties of a lubricating oil over the use of any of the viscosity modifiers used singly.
Previous methods used to solve this problem have involved judicious selection of base oils and viscosity modifiers. Often, though, low temperature fluidity must be achieved by using very thin base oils, and the high temperature viscosity requirements are then met with the use of viscosity modifiers. The tightening of the shear stability requirement has severely limited the choice of viscosity modifiers because many are not capable of meeting the shear requirements.
U.S. Pat. No. 6,133,210, Tipton, discloses concentrates for preparing lubricating oil compositions such as automatic transmission fluids. The viscosity index improver can be at least one of a polyacrylate ester copolymer, optionally containing nitrogen-containing groups; and an esterified carboxy-containing interpolymer, where one of the monomers is a vinyl aromatic monomer and the other monomer is an alpha, beta-unsaturated acylating agent. The polyacrylate ester can have a Mn of 20,000 to about 100,000. The esterified carboxy-containing interpolymer can have an RSV of 0.05 to 0.35.
U.S. Pat. No. 6,124,249, Seebauer et al., discloses viscosity improvers for lubricating oil compositions, being a copolymer with units of methacrylic acid esters containing 7–12 and 13–19 carbon atoms in the ester group; and a comonomer that can be dimethylaminopropylmethacrylamide. Auxiliary viscosity improvers can also be included in the lubricating composition, including esterified styrene-maleic anhydride copolymers (col. 16). The molecular weight ( Mw) of the acrylic copolymer is listed as 20,000 to 120,000
U.S. Pat. No. 5,646,099, Watts et al., discloses an automatic transmission fluid of improved viscometric properties, containing (among other components) 0.05 to 2.0 weight percent of a flow improver selected from the group consisting of C8 to C18 dialkylfumarate vinyl acetate copolymers, styrene-maleic anhydride copolymers, polymethacrylates, polyacrylates, and their mixtures.
The present invention, therefore, solves the problem of providing low temperature fluidity, high temperature viscosity and shear stability in a fluid by using two types of viscosity modifiers: a polymethacrylate ester and an ester of a maleic anhydride/styrene copolymer. Such fluids are useful in a variety of climatic conditions.
The fluids of the present invention are advantageously used as tractor hydraulic fluids and can also be used as a variety of other functional fluids, including manual transmission fluids, automatic transmission fluids (including fluids for continuously variable transmissions and traction drives) and other hydraulic fluids. They can also be used in other lubricating applications such as gear oils and engine oils.